Modulating system



` Feb. 26, 1946.

R. E. MEYER FIG.I

MODULATING SYSTEM Filed June 14, 1943 ouTPuT MODULATING 2| ,9 25` SIGNAL souRcE @E 6 a@ 2 E .L

22 l2 I [L [L AIHIHIHIAIHIHI '3fm l5' 27 m O O I CARRIER /Il wAvE GENERATOR /ll Y /`29 INVENTOR RAY ND E. MEYER Patented Feb. 2.6, 1.9.46

2,395,573 MonULA'rlNG SYSTEM Raymond E.vMeyer, Fort Wayne, Ind., assignor to Farnsworth Television and Radio Corporation, a corporation oi' Delaware Application June 14, 194s, serial No. 490,752

2 Claims.

This invention relates to modulating systems and particularly to apparatus for stabilizing the frequency of an amplitude modulated wave.

According to conventional practice, a frequently employed means for modulating a carrier wave in amplitude in accordance with a modulating signal comprises a grid modulated power amis provided a carrier wave generator and means plifler arranged for so-called class C operation. t

In. such a system the grids of the power amplifier tube are coupled to a carrier wave generator for excitation thereby. The grid circuits also are provided with a relatively large negative bias so that grid current flows only during portions ci the alternating current cycles of the carrier wave. The time during whichthe grid current ows is controlled by the strength of the modulating signal. The flow of grid current in the power amplifier results in a power drain upon the carrier wave generator which varies in proportionto the strength of the modulating signal.

As is well known in the art, however, the power drain upon a carrier wave generator such as an oscillator is reflected into the frequency determining circuit thereof. Such reflection is manifested in this circuit as a resistance. Consequently, a variation in the energy derived from the carrier wave generator effects a variation in the eiective resistance of the frequency determining circuit. The frequency cfa tuned circuit is given exactly by the equation cuit. Since the frequency of the carrier wave is thus a function of the resistance of the frequency .determining circuit, a variation in the effective resistance of this circuit produces a corresponding variation in the frequency of the generated carrier wave. Thus, in the output circuit of the grid modulated power ampliiier there appears a carrier wave which is modulated, not only in am. plitude as desired, but also in frequency. The latter type of modulation is undesirable and in some cases may be so great as to render the operation of the system unsatisfactory.

It is an object of the Present invention, therefore, to provide, in an amplitude modulating system, means controlled by the modulating signal for preventing frequency modulation of the carrier wave.

Another object of the invention is to provide for deriving energy therefrom for excitation pur poses which is directly proportional to a modulating signal. There also is provided a means for deriving additional energy from. the generator which is inversely proportional to the modulating signal. Thus, the load lwhich is placed upon the carrier Wave generator by the derivation therefrom of the two portions of energy is maintained constant.

More specifically, in accordance with the em bodiment of the invention illustrated herein, there is coupled to the generator the input circuit ci a power amplifier. There also is connected to the input circuit ofthe power ampliiier a modulating signal, whereby t0 vary the amount of energy derived from the generator to drive the power amplifier in accordance with this signal.

Additionally, there is provided a variable com pensating load circuit which also is coupled to the carrier wave generator. is derived from the generator by the compensating load circuit is controlled by the modulat= ing signal. However, the modulating signal is connected to the compensating load circuit in phase opposition to its connection to the power amplifier. Thus, the energies derived from the generator by the power amplier and the com pensating load circuit are in phase opposition in these two circuits so that a constant load is main tained on the generator.

For a better understanding ci the invention, together with other and further objects thereof, reference' is had to the following description, taken in connection with the accompanying drawI ing, and its scope will be pointed out in the -ap pended claims.

In the accompanying drawing:

Fig. l is a circuit diagram of one embodiment of the invention; and,

Fig. 2 is a graphical representation of the en-l ergy components derived from the carrier wave generator which will be referred to in the explanation of the operation of the disclosed cir cuit.

Referring now more particularly to Fig. 1 of the drawing, there is shown a carrier Wave gemI erator il which, forexample, may be any conventional type of oscillator. A parallel tuned cir cuit comprising an inductor l2 and a condenser The energy which t I3 is connected to the output terminals of the generator. The parameters of this circuit are adjusted to effect resonance of the circuit at lthe frequency of the generator.

An inductor I4 is coupled to the generator inductor I2 and its terminals are connected to the control grid electrodes of a twin triode electron discharge device I5. A condenser I6 also is connected to the terminals of the inductor I4, whereby to tune the parallel circuit so formed for resonance at the frequency of the carrier wave generator I I. The anodes of the vacuum tube I are connected to a parallel resonant circuit including a condenser II and an inductor I8. This circuit also is tuned for resonance at the frequency of the generator II. Acoil I9 is coupled inductively to the inductor IB and is connected to the terminals 2I of an output circuit. The tube I5 and its associated input and output circuits comprise a power amplifier. Conventional neutralization (not shown) of the power amplier preferably is employed but has been omitted in order to sirnplify the drawing.

There also is provided an .inductor 22 which. is coupled to the generator inductor I2. A tuning condenser 23 is connected to the terminals f the inductor 22, whereby to effect resonance of the parallel circuit so formed at the frequency of the generator II. The terminals of the inductor 22 are connected to the anodes of a double diode 24.

A source of modulating signal 25 is connected to the input circuits of the tube I5 and also of the diode 24. The connection of the modulating signal source to the grids of the tube I5 includes a source of negative biasing potential suchas a battery 26, whereby the tube is arranged for so-called class C operation. The connection of the modulating signal source 25 to the diode 24 cordance with this invention, this varying resistance would eifect a variation in the frequency of the carrier wave generated.

However, there is provided in accordance with the present invention the compensating load circuit including the coil 22 and the diode 24. The energy derived from the generator I I for this circuit is determined -by the conductivity of the tube 24. The conductivity of this tube is controlled by the modulating signal from the source 25. Thus,

in the assumed case of a sinusoidal modulating signalI the conductivity of the tube 24 and the energy derived from the generator II varies sinusoidally. But in the case of this circuit, it is to be noted that the modulating signal is applied to the diode 24 in phase opposition to the application thereof to the amplier tube I5. Thus, as the energy supplied through the 'coil I4 is lncreasing, the energy supplied through the coil 22 is decreasing correspondingly.

This relation may be observed by referring to Fig. 2 wherein the broken line curves 29 represent the envelope of the carrier frequency energy which is derived from the generator I I by the coil 22 and the diode 24. yIt is seen that these curves are displaced 180 electrical degrees from the curves 28. By suitable adjustment of the diode is in phase opposition to the connection of this source to the amplifier tube I5.

Space current for the ampliiier tube I5 and the carrier wave generator II is supplied from a source of direct current potential such as a battery 2l. These connections are made by way of center-taps on the inductors I2 and I8.

Referring now to the operation of the apparatus described, consider first the manner in which the amplitude modulation of the carrier wave is effected. The grids of the tube I5 are excited at the carrier wave frequency by means of energy derived from the carrier wave generator I I through the coil I4. The amount of energy so derived is determined by the modulating signal supplied from the source 25. For the purposes of this explanation, assume that the wave form of the modulating signal is sinusoidal. When such a signal is applied to the grids of the tube l5 the bias thereof varies sinusoidally, and, depending upon the instantaneous amplitude of the signal, the grids of the tube draw varying amounts of current through the coil I4. This places a load upon the carrier wave generator II which varies sinusoidally.

In Fig. 2, the solid line curves 28 represent the variation of the carrier Wave energy which is derived from the generator I I as the modulating signal varies sinusoidally. The average power which is supplied by the generator to the power amplifier over a period of time is represented by A. However, by reason of the continuous variation of the power supplied by the generator there is reiiected into the output circuit thereof a continuously varying resistance. If it were not for the stabilizing arrangement provided in acl ent, is considered the circuit parameters, the maximum and minimum amplitudes of the curves 29 may be made equal to corresponding amplitudes of the curves 28. In `this case the average power supplied by the generator I I to the compensating load circuit including the diode 24'is represented by "A," also. However, the instantaneous amplitudes of the curves 29 are continuously varying in phase opposition to the variations of the instantaneous amplitudes of the curves 28. The respective instantaneous amplitudes at all times are mutually complementary so that the total energy derived from the generator is maintained constant. The

total energy supplied bythe generator is represented at B.

As a consequence of this arrangement, the tendency produced by the derivation of the varying amounts of power by the power amplifier to effect resistance variations in the frequency determining circuit of the generator II is counteracted by a similar tendency in an opposite sense to produce resistance variations by the derivation of varying amounts of power by the'compensating load circuit.

Thus, there is produced a result whereby to effect a complete cancellation of the eiiects which would produce resistance changes in the output circuit of the carrier wave generator l I. As a consequence, the eifective resistance of this circuit is maintained constant, whereby the frequency `of the carrier Wave also is maintained constant in so far as such frequency is dependent upon the elfective resistance.

While there has Vbeen described what, at prespreferred embodiment of the invention, it is obvious to those skilled in the art that various changes and modincations may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. In a modulating system, a carrier wave generator, a power amplifier including an electron dischargedevice having a pair of control grids, means for coupling inductively said control grids to said generator for excitation thereby, a comrsalii) means including a coil for coupling inductively said control grids to said generatorl for excitation thereby, a compensating load circuit including a full wave rectifier, means including a second coil for coupling inductively said rectifier to said generator, and a source of modulating signal connected to the input circuits of said electron discharge device and said rectiiier in phase opposition. said modulating signal serving to grid-modulate said power amplifier and simultaneously to control conduction in said rectier, whereby to maintain a constant load on said generator.

RAYMOND E. MEYER. 

